Hydrocarbons (oil, natural gas, etc.) are typically obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. A typical well is drilled as a vertical well into the subsurface. However, in recent times drilling practice evolved to include drilling of highly deviated (from the vertical) or horizontal wells to improve the contact of the well with a specific formation layer or pay zone.
Prior to production, the well is completed with the installation of production facilities, such as casing, production pipes and pumps. The hydrocarbon industry distinguishes between two basic types of completion. One is referred to as “open hole” completion characterized by leaving the drilled well without a casing lowered into the well in the formation zone and without a consolidating sheath of cement squeezed into the space between the drilled formation and the casing. If, on the other hand, the well is completed with casing and cement in the formation zone, the completion is referred to as “cased hole”.
In order for hydrocarbons to be “produced,” that is, travel from the formation to the wellbore (and ultimately to the surface), there must be a sufficiently unimpeded flowpath from the formation to the wellbore. This flowpath is through the formation rock, e.g., solid carbonates or sandstones having pores of sufficient size, connectivity, and number to provide a conduit for the hydrocarbon to move through the formation.
One key parameter that influences the rate of production is the permeability of the formation along the flowpath that the hydrocarbon must travel to reach the wellbore. When a hydrocarbon-bearing, subterranean reservoir formation does not have enough permeability or flow capacity for the hydrocarbons to flow to the surface in economic quantities or at optimum rates, hydraulic fracturing or chemical (usually acid) stimulation is often used to increase the flow capacity.
Hydraulic fracturing consists of injecting viscous fluids (usually shear thinning, non-Newtonian gels or emulsions) into a formation at such high pressures and rates that the reservoir rock fails and forms a plane, typically vertical, fracture (or fracture network) much like the fracture that extends through a wooden log as a wedge is driven into it.
Granular proppant material, such as sand, ceramic beads, or other materials, is generally injected with the later portion of the fracturing fluid to hold the fracture(s) open after the pressures are released. Increased flow capacity from the reservoir results from the more permeable flow path left between grains of the proppant material within the fracture(s).
In chemical stimulation treatments, flow capacity is improved by dissolving materials in the formation or otherwise changing formation properties. The acidizing fluid is disposed within a well drilled into the formation to be fractured. Sufficient pressure is applied to the acidizing fluid to cause the formation to break down with the resultant production of one or more fractures therein. An increase in permeability is effected by the fracture formed as well as by the chemical reaction of the acid within the formation.
In a variation of the method involving acidizing, the formation is first fractured. Thereafter, an acidizing fluid is injected into the formation at fracturing pressures to extend the created fracture. The acid functions to dissolve formation materials forming the walls of the fracture, thus increasing the width and permeability thereof.
Fracturing is a very well established method and described in an extensive body of literature. Among those seen as most relevant to the present invention are U.S. Pat. No. 2,970,645 issued to Glass, U.S. Pat. No. 4,718,490 issued to Uhri, U.S. Pat. No. 4,867,241 issued to Strubhar, U.S. Pat. No. 4,883,124 issued to Jennings, U.S. Pat. No. 4,917,185 issued to Jennings et al., U.S. Pat. No. 4,951,751 issued to Jennings, U.S. Pat. No. 4,974,675 issued to Austin et al., U.S. Pat. No. 4,977,961 issued to Avasthi, U.S. Pat. No. 5,161,618 issued to Jones et al., U.S. Pat. No. 5,238,067 issued to Jennings, U.S. Pat. No. 5,507,342 issued to Copeland et al., U.S. Pat. No. 6,543,538 issued to Tolman et al., U.S. Pat. No. 6,719,054 issued to Cheng et al., U.S. Pat. No. 7,004,255 issued to Boney, U.S. Pat. No. 7,028,775 issued to Fu et al., and U.S. Pat. No. 7,148,184 issued to Francini et al. These patents disclose fracturing methods, as well as acid and proppant compositions. In particular, the '645 and '067 patents relate to methods of creating multiple fractures.
In the view of the above referenced patents it is seen as an object of the present invention to provide novel methods of creating multiple fractures, particularly multiple fractures in highly deviated or horizontal wells with open hole completions.